Collision on the cable

Hello, I want to ask a question that is confusing me.
The UTP cable is designed of 8 little cables, 2 for transmit and 2 for recieve.
If 2 for transmit and 2 for recieve and they are independant of each other, then why there will be a collision if the two devices sends at the same time????

The second question is that what is the purpose of the other 4 little cables.

As you said, there are two for RX (RX+ and RX-) and two for TX (TX+ and TX-). Blue and brown are unused (PoE takes advantage of these). From memory, though, blue is used for ground in both PoE mode and normal mode.

On a hub (not a switch) when 2 pcs send out traffic it is replicated to every port on the hub. When 2 PCs send out traffic at the same time, the hub detects a collision and each PC will re-transmit the packet after a random length of time.

Sorry, but the quetion isn't answered yet.
The main idea, is that the UTP cable consisted of 8 little cable. 2 of them are for transmit, and 2 for recieve,then how is the collision occurs??!!! (The collision occurs if the transmit and the recieve are on the same line).

I get where you're coming from. There is no collision in the sense of two NICs TXing on the same lines at the same time, but there is collision in that if both NICs are TXing and neither is in RX, waiting for data, then neither of the NICs will receive any data, because they weren't in RX mode.

There are 2 wires each for tx and rx because the hardware uses what are called line-drivers to move the signal. This minimizes interference and increases distance for the transmitted signals and greatly increases the reliability of the media. FWIW, you CAN transmit and receive at the same time, if the system is set up for full-duplex operation. Most modern switches and NICs allow this, although you can configure them for half-duplex operation, where you can only send or receive at any particular time. If one end is configured full-duplex and the other half, then the thruput is greatly diminshed.

CimmerianX is correct about hubs vs. switches, which is why most networks these days use switches to eliminate (or greatly reduce) collisions.